My 2004 Cooper S warms up to normal operating temp like normal. About 5 minutes after reaching normal temp, gauge at the halfway point, the cooling fan comes on, the gauge rises rapidly and the overheat light comes on. After I shut it down the fan continues running and after about a minute the gauge lowers back down fairly rapidly to normal. There is no coolant loss. The oil is not milky. There were previous cooling issues. Any ideas?

 

Bad thermostat, water pump, or blocked radiator?

 

Agree with above... But thermostat is most likely the cause. You could open the bleeder near the radiator as it is heating up. It usually will not bleed much untill the thermostat opens. If it does not start to bleed once up to Temp (and beyond)... I'd start with the thermostat.

Just my .02

 

Need to do some more diagnosis to see what temp it's getting up to. And check for coolant flow. Also a real temp gauge will show if the thermostat is opening as you will see the temp on the sensor drop when it does.

 

Bad diagnosis on my part. PTO went out. Thanks for all the help.